1. Field of the Invention
The present invention is related to the field of data processing methods for oil well logging. More specifically, the present invention relates to methods for improved formation data evaluation by azimuthal tool path identification.
2. Background Art
The petroleum industry uses various tools to obtain measurements for estimating earth formation properties. Typical measurements include density, porosity, and conductivity. These measurements are often used in combination to derive the formation properties. For example, the formation density is often combined with other measurements (e.g., neutron porosity measurements and resistivity measurements) to determine gas saturation, lithology, porosity, the density of hydrocarbons within the formation pore space, properties of shaly sands, and other parameters of interest.
Most of these measurements can be performed either after a borehole has been drilled or simultaneously with the drilling of the borehole, i.e., logging-while-drilling (LWD) or measurement-while-drilling (MWD). Regardless of how these measurements are performed, they are generally sensitive to environmental effects such as the position of the logging tool in the borehole and the physical properties of the drilling fluid. For example, in neutron porosity measurements, the size of the borehole, the amount of stand-off of the tool from the borehole, the hydrogen index and salinity of the drilling fluid, and the salinity of the formation fluids, to name just a few, all affect the accuracy of the measurements. Similarly, in gamma-gamma density measurements, which have relatively shallow depths of investigation but are strongly focused, stand-off and drilling mud could have significant impact on the accuracy of the density measurements. Therefore, accurate formation evaluation depends on minimizing these environmental effects and/or correcting for them.
These environmental effects and their corrections can be best illustrated in gamma-gamma density logging. J. S. Wahl, J. Tittman, and C. W. Johnstone introduced a method of density measurement using an isotopic gamma ray source (e.g., 137Cs) and two gamma ray detectors (scintillation counters), in xe2x80x9cThe Dual Spacing Formation Density Logxe2x80x9d, Journal of Petroleum Technology, December 1964. The basic concepts disclosed by Wahl et al. are still in use today. The density logs thus obtained are often referred to as dual spaced density logs or gamma-gamma density logs. The dual spacing formation density log is obtained using a tool having a gamma radiation source (e.g., 137Cs) and two detectors (scintillation counters), one at a shorter distance and the other at a longer distance from the gamma radiation source. The apparatus is configured as a logging tool (sonde) for xe2x80x9cloggingxe2x80x9d formation density as a function of depth along a borehole. The source and two detectors are typically mounted in an articulating pad device with a backup arm. The backup arm applies force to the articulating pad to maximize pad contact with the wall of the borehole. Special protective shields are installed so that the detectors do not detect gamma radiation directly from the source. Instead, the detectors measure radiation that has been scattered by the formation into the detectors. The scatter reaction is primarily Compton scattering, and the number of Compton scattering collisions within the formation is related to electron density of materials within the formation. Through calibration, a measure of electron density of the formation can be related to true bulk density of the formation.
As stated earlier, the gamma-gamma density measurements, which may be output as bulk density, compensated bulk density, correction factor (xcex94xcfx81), or photoelectric absorption cross section (Pe), are adversely affected by tool stand-off and mud existing between the tool and the borehole wall. By using two detectors, a short space (SS) detector and a long space (LS) detector, Wahl et al. were able to correct for these effects from raw density measurements. This correction is based on the phenomenon that the shorter the spacing, the shallower the depth of investigation and the larger the effect of the mudcake. Thus, a short spaced detector, which is very sensitive to the mudcake, can be used to correct a long-spaced detector, which is less sensitive to it. The measurements from the short-spaced and long-spaced detectors are often analyzed with spine-and-ribs plots, which are plots of long-spacing versus short-spacing count rates.
The spine-and-ribs plot takes its name from the appearance of a spine, which is the locus of points with no mudcake, and ribs, which show the effect of mudcake at certain fixed formation densities. In principle, three major factors influence the count rates: formation densities, mudcake densities, and mudcake thicknesses. The spine-and-ribs plot illustrates graphically that for a given formation density there is only one rib for all normal mudcake densities and thicknesses. Thus, although there are three unknowns, it is possible to make a correction using two measurements.
While the spine-and-ribs plot can afford approximate corrections for the mudcake effects, corrections become problematic in an LWD/MWD environment in cases where the rotating tool is slick, under-gauge, or nominally in-gauge but passing through a wash-out. In these instances, stand-off and hence the amount of drilling fluid between the detectors in the tool and the formation depends on the tool""s orientation in the borehole, and this varies as the tool rotates. Historically, measurements have been averaged over this azimuthal direction and corrections for the tool/borehole environment applied explicitly and uniformly. This is still the case in traditional neutron porosity processing. This procedure combines measurements taken in different physical configurations (along the azimuthal direction) with only a rough correction for the environment; hence, it can give inaccurate results.
With the advent of the ability to make azimuthal measurements (i.e., measurements in azimuthal sectors at a depth level) while drilling, the situation has improved but is not yet ideal.
Gamma-gamma density measurements can now be acquired in azimuthal sectors. These azimuthal density tools (e.g., azimuthal density neutron tools produced by Schlumberger under the trade name of ADN(trademark)) can provide measurements that are borehole compensated for improved accuracy, standoff, and photoelectric factor measurements while drilling. Stand-off and mud weight corrections are applied implicitly to measurements from these azimuthal density tools on a sector-by-sector basis with a spine-and-ribs approach. This technique improves accuracy. However, most such tools provide appropriate corrections only out to approximately 0.5 inch of stand-off. For a slick 8.25 in. tool in a 12.25 in. borehole, up to 4.0 in. of stand-off occurs routinely. Consequently, many sectors of measured density are inaccurate.
The azimuthal tools produce arrays of formation density measurements at each depth or time level, some of which are more accurate than others. However, it is often desirable that a single density, which characterizes the formation, is available at a given depth or time level. To satisfy this need, one approach is to assume that the tool""s most accurate measurements are obtained when the detectors are oriented toward the bottom of the borehole. In a deviated borehole under ideal conditions, the bottom sector has a minimal distance between the detectors and the borehole wall. Under this assumption, measurements from the sectors in the bottom quadrant are combined to provide a single formation density. This technique can be used in azimuthal tools (such as the ADN(trademark) tools) and offers a substantial improvement over the averaging technique.
However, rotation during drilling and changes in borehole trajectory can cause the most accurate orientation to shift from the bottom of the hole. The same situation can occur around sharp dog-legs or if the borehole becomes rugose or elliptical in cross section. Clearly, formation measurements in slick or under-gauge situations would be improved if the orientation of the rotating tool providing the most accurate measurement were known and the data from the sectors surrounding this orientation were used to derive the final result.
Much effort has been directed at obtaining improved formation evaluation, particularly density, based on selecting those sectors with more accurate measurements. None of these efforts completely addresses the problem, however.
U.S. Pat. No. 5,091,644 issued to Minette discloses a method for minimizing error in formation data in a MWD logging tool by comparing the relative error of measured spectra from different azimuthal sectors and combining those spectra in a way that minimizes the error. The relative error may be a combination of rib error and statistical error. The sectors may be defined by an accelerometer and/or a magnetometer with respect to time or by a caliper with respect to stand-off. Four difficulties exist with this approach. First, in situations with large stand-off or large washouts, the measured density has a small relative error but is characteristic of the drilling fluid, not the formation. Thus, relative error is not the same as measurement quality. Second, the relative error considered involves mainly stand-off and statistical uncertainty but neglects the equally important mud weight effects. Third, dipping beds pose a problem for this approach, because relative error can give misleading formation densities in these cases. Fourth, the stand-off measurements typically required with this technique may be unreliable in large holes or heavy muds.
U.S. Pat. No. 5,397,893 issued to Minette discloses a method for minimizing error in formation data in a MWD logging tool based on binning the signals received by the tool based on measured stand-off values. This approach has similar difficulties.
U.S. Pat. No. 6,307,199 B1 issued to Edwards et al. discloses methods for compensation of geometric effects in logging-while-drilling density measurements. These methods are for determining a one-dimensional density, corrected for stand-off and dipping beds penetrated by a borehole, through an analysis of a two-dimensional image from combined depth and azimuthal measurements of density. This technique relies on measurements from several depth levels and is therefore best used in recorded mode, not in real time.
U.S. Pat. No. 5,473,158 issued to Holenka et al. discloses a method for correction that can be used in logging while drilling and apparatus for measuring formation characteristics as a function of angular position within a borehole.
One aspect of the invention relates to methods for tool path identification. Methods for tool path identification according to embodiments of the invention include obtaining measurements of formation properties in azimuthal sectors for each of a plurality of depth levels; calculating quality factors from the measurements; identifying a maximum of the quality factors among the azimuthal sectors at each depth level; and associating the maximum of the quality factors at each depth level along a borehole to form the tool path. The calculating the quality factors may include parameterizing the measurements according to at least one factor selected from a spine factor, a rib factor, and a volumetric photoelectric factor.
Another aspect of the invention relates to methods for determining formation properties through tool path identification. Methods according to embodiments of the invention include identifying a tool path from measurements taken in azimuthal sectors at each depth level along a borehole; and computing corrected measurements at each depth level by averaging measurements in a selected number of azimuthal sectors adjacent the tool path.
Another aspect of the invention relates to methods of well logging. Methods according to embodiments of the invention include moving a well logging tool along a borehole and rotating the tool; making measurements in azimuthal sectors at each depth level; determining a tool path from the measurements in the azimuthal sectors; and computing corrected measurements at each depth level by averaging the measurements in a selected number of azimuthal sectors adjacent the tool path.
Other aspects of the invention would become apparent from the following description, the drawings, and the claims.